Field of the Invention
The invention relates generally to optical devices. In particular, the invention relates to silicon based optical modulators.
Description of the Related Art
Optical modulators are the key component in optical communication systems. Optical modulators are devices that convert electrical signals to optical signals. An optical modulator is traditionally made of single crystal materials, such as lithium niobate (LiNO3) or III-V semiconductors that have strong electro-optic effects. However, devices made of these materials tend to be expensive and are mostly deployed in telecommunication systems.
In recent years, high speed data communication becomes an emerging direction of optical communication. Data communication systems are very sensitive to cost. As a key component, optical modulators employed in data communication systems have to be low cost. Silicon photonics is an emerging technology that could provide low cost solutions for data communication systems. Naturally, a low cost silicon based optical modulator is highly demanded.
Since silicon is a very “passive” material, the only effect that can be used for optical modulation so far is the free-carrier effect. Silicon modulators based on free-carrier effect have been extensively studied in the past decade. Among them, modulators utilizing reverse biased PN diodes have been a promising approach to realize low cost high speed modulation. Under reverse bias, the depletion region of the PN diode junction enlarges, which results in a refractive index change of the waveguide and in turn optical phase change. Laterally oriented PN diodes are mostly employed due to the relatively simple fabrication process. However, small overlap between the depletion region and optical mode of the waveguide limits the modulation efficiency. On the other hand, vertically oriented PN diodes can provide higher modulation efficiency at the cost of more complicated process. In this case, the key is to reduce the optical loss induced by the high doping regions that is used to improve the ohmic contact between the metal electrodes and the silicon materials. The only way to achieve this is to position the high doping regions outside the light propagation region to reduce series resistance and maintain high speed performance, which makes the processes much more complicated. There is a need to well balance the optical loss and high speed performance of such devices without increasing the complicity of the fabrication process.